This invention relates to co-channel multiple signal broadcasting mobile communication system and more particularly to such a co-channel multiple signal broadcasting system with interference-free wide coverage.
Communication coverage over an extended geographical area such as is present in a mobile communication system can be achieved by using several transmitters to broadcast the same message on the same carrier or channel. This technique is finding acceptance in urban, suburban or country-wide areas where surface terrain preclude the use of a single transmitter. This simultaneous broadcasting, however, causes the communiction environment within the extended coverage area to be interference limited rather than front end noise limited.
Interference zones are created whenever overlap coverage exists, since two or more signals at nominally the same frequency may arrive at comparable amplitude levels, but with different modulation or group delays. The result can be interference that is similar to that produced in a multi-path environment. With the limied capture performance of commercial receivers the end result may well be that the extended area of coverage is interlaced with zones where the receive signal becomes unintelligible.
The reduction of the interference in overlap areas to tolerable levels is the problem solved by the present invention and which in the past has been solved by other techniques.
One of the prior art techniques to reduce distortion in overlap areas is the use of site selection, radiated power and directional antennas to minimize the size of overlap areas. This approach has only limited value since site selection is often dictated by convenience rather than RF (radio frequency) path suitability. In any case, minimizing overlap areas concurrently reduces the radio link safety margin within affected zones.
A second technique employed in the prior art to reduce distortion in overlap areas is to improve the receiver capture ratio. This can be accomplished by modifying commerical receivers or by manufacturing a new line of receivers with improved performance.
A third technique suggested in the prior art to reduce distortion in overlap areas is the use of a steerable antenna at the receiver sites. At present no commercial units with automatic steering are available for mobile applications.
A fourth technique employed in the prior art to reduce distortion in overlap areas synchronizes carrier frequencies and modulation phase delays in the overlap area. This technique has limited application in cities, mountainous terrain or in locations where transmitter spacing is more than 15 miles.
A fifth technique to reduce distortion in overlap areas separates the transmit carriers of the interfering transmitters on a static basis sufficiently to suppress the interference. This technique is known within the industry as "tertiary offset" and/or "territory offset". The problem with this technique is that more spectrum is required, FCC (Federal Communication Commission) regulations must be changed and wider bandwidth receivers are needed.